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Unformatted text preview: pg1of8 EK307 —- Electric Circuit Theory -— Fall 2007 Final Exam
0 This exam consists of three parts:
1) Basic circuit theory questions; 2) RLC circuit problems, and 3) Lab questions.
0 You have two hours to complete this exam
0 Up to three sheets of notes plus calculator permitted. No other books or materials allowed.
You may use your lab notebook for Part 3.
0 Put all answers in the boxes or on the graph axis provided. Credit will only be given for
answers entered in the proper place. Part I — Ten Short answer questions _
Write the letter of the answer in the box provided. 1. Two 1.5-V D—cell batteries are used to power a small light bulb. The brightest light will be obtained by connecting the elements in which way?
Connect the cells in series with the bulb. Connect the cells in anti—series with the bulb. Connect the cells in parallel with the bulb:
Connect the cells in anti-parallel with the bulb.
Series or parallel makes no difference to bulb brightness as long as both cells are used. 2. An ohmmeter is connected to the circuit shown below at terminals A and B. The ohmmeter reads 259
509 509 509 A
759
1009
1509 B 3. Which term describes the number of electrons ﬂowing per unit time? Resistance Power
Capacitance Voltage
Current 4. How is an ammeter usually connected to a circuit under test?
In series with the circuit.
In parallel with the circuit.
In series and in parallel with the circuit.
In phase with the circuit.
With a coaxial clip—lead test cable. 5. A 4800-52 resistor is connected to a voltage source of value 120 cos cot V, where 0) = 2n x 60 Hz. What is the power dissipated in the resistor?
3.0 W 4.2 W
2.1 W 69.mW
1.5 W ngof8 6. If three resistors are connected in parallel, their combined resistance is
Equal to the average value of the individual resistors.
Always less than the value of any one resistor.
Always larger than the value of any one resistor.
Equal to the product of the three resistors.
Equal to the sum of the three resistors. 7. What are the physical units of the product RC, where R: resistance and C: capacitance?
Voltage
Current
Time
Power
Frequency 8. What is the resistance value seen between terminals A and B? 259 1009 509
‘679 A 1009 1509 509 1009
3009 B 9. What is the voltage vx? 100 Q 50 Q 1V 4V 12v 9 6V 3V 4.5V +vX_ 10. A 10-pF capacitor is precharged to 10 V by a battery. At t = 0, the battery is disconnected, and a l-kQ resistor is connected across the capacitor's terminal. After 10 ms, the capacitor has
...been discharged to a voltage of about 3.7 V
...been discharged to a voltage of about 6.3 V
...been discharged to 10 mV
...been discharged essentially to zero
...retained most of its lO-V charge. The resistor is too small to
signiﬁcantly discharge the capacitor in a few milliseconds. pg 3 of 8
Part II - RLC Circuits (20 points each) 11. Consider the circuit shown below. a) Draw the Thévenin equivalent of everything connected to the diode.
Specify the values of VTh and RTh. pg 4 of 8
12. Consider the circuit shown below. It has been energized for a long time prior to t = 0. +5V R = kg
01:2; ”F ‘v 3
OUT a) What is the value of v1 for t < 0?
Vow-r; b) What is the value of v1 for t > 0 after many time constants of the circuit have elapsed? c) Plot v1(t) for t > 0 on the axes provided. Label the voltage and time scales. pg5of8 13. The circuit shown below is designed to operate an alarm when the current through R2
exceeds 0.5 mA. R2=4kQ :: ————————————————————————————— a) Find the circuit time constant for t _>_ 0+ b) Find the time needed for the alarm to turn on. pg7of9 14. In problem, we explore the complex impedance Zn of the following circuit: CI: 2500 pF a) Express Zn in terms of R1, R2, C1, C2, L, and b) If Zn = A + j B, ﬁnd numbers for A and B if 0) = 50 rad/sec. pg7of8 15. Consider the parallel RLC circuit shown below. The circuit is pre-energized with the switch
closed. The switch is then opened at t = 0. c) Is the circuit under-damped or over-damped?
under-damped over—damped d) What IS the‘initial condltlon for the voltage derivatlve LC at t = 0+? ...
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